The present invention relates to a control device for an electric injection molding machine.
In a conventional electric injection molding machine designed to advance and retract a screw by driving an injection motor, the screw is advanced in an injection process to inject a molten resin, a molding material which has been heated in a heating cylinder, into a cavity of a mold under a high pressure to fill the cavity with the molten resin. The resin in the cavity is cooled and caused to set to obtain a molded product part. Subsequently, the mold is opened to remove the molded product part.
In that case, the screw speed is changed in the injection process each time the screw has reached a predetermined position.
FIG. 1 is a schematic view of a conventional electric injection molding machine.
In FIG. 1, a ball screw 12 is rotated by driving an injection motor 11. The ball screw 12 is threaded with a ball nut 13 formed integrally with a supporting member 14, which is in turn mounted in such a manner as to be movable along guide bars 15 and 16 mounted on a frame (not shown).
Thus, when the ball screw 12 is rotated by driving the injection motor 11, the supporting member 14 moves along the guide bars 15 and 16. The movement of the support member 14 is transmitted to a screw 20 through a load cell 18 and a bearing 19.
When the screw 20 is advanced in a heating cylinder (not shown) in the injection process, the molten resin located at the front end portion of the heating cylinder is injected into the cavity of a mold (not shown). At that time, as the screw 20 presses against the resin, a reaction force is generated and applied to the screw 20. Hence, a load cell 18 detects this reaction force, and a load cell amplifier 21 amplifies the output of the load cell 18 and inputs the amplified output to a controller 22.
To detect screw position, a screw position detector 23 is mounted between the support member 14 and the frame. An amplifier 24 amplifies the screw position detection signal of the screw position detector 23, and inputs the resultant signal to the controller 22. The controller 22 outputs a speed instruction signal determined for every process on the basis of the operator's setting value input to a servo amplifier 25 to drive the injection motor 11.
That is, to control the screw speed and dwell pressure during molding, the screw position is fed back in the injection process in which the resin is injected into the cavity of the mold, while the dwell pressure, the reaction force detected by the load cell 18, is fed back in the dwelling phase in which a fixed amount of pressure is applied to the resin filled in the cavity of the mold.
In a conventional controlling method, when the screw 20 is advanced in the injection phase, the screw speed is changed stepwise at a plurality of screw positions.
FIG. 2 is a block diagram of a control device for a conventional electric injection molding machine.
In FIG. 2, when the operator inputs changing positions and injection speeds to a position pattern generator 28 of the controller 22 from a setting unit, the position pattern generator 28 performs calculation on the basis of the set values, and generates a position setting pattern signal a. The position setting pattern signal a is generated on the basis of the relation between the changing positions and the screw speeds desired by the operator, and consists of a time signal and a screw position instruction signal.
The position setting pattern signal a is output to a subtracter 29 to which an actual screw position detection signal d is fed back from the amplifier 24. The subtracter 29 outputs a position deviation signal b to a compensator 30. The compensator 30 performs a compensation operation, and outputs a speed instruction signal c to the servo amplifier 25. In this way, the screw speed is controlled by feeding back the screw position.
In the thus-arranged control device for the electric injection molding machine, even when a normal proportional control is performed on the basis of the position setting pattern signal a, a steady-state speed deviation or an acceleration deviation may occur between the set position and the actually detected screw position, because responsiveness is low.
Thus, the operator who intends to change the screw speed by the position setting pattern signal a may not be able to change the screw speed at a desired changing position.
Consequently, the screw speed may be changed before the screw 20 (FIG. 1) has reached the operator's desired changing position.
Hence, it may be provided such that the compensator 30 contains the integration element to decrease the steady-state speed deviation or the acceleration deviation. However, this may generate overshoot or vibrations in response.
After the resin has been injected into the cavity of the mold and the injection phase has thus been completed, the dwelling phase in which the resin pressure is maintained at the set value to compensate for the shrinkage of the resin caused by cooling.
An electric injection molding machine designed to control the dwell pressure will now be described.
FIG. 3 shows another type of conventional electric injection molding machine.
In FIG. 3, an output shaft 35 of the injection motor 11 is coupled to a pulley 36, which is in turn coupled to a pulley 38 through a timing belt 37. A ball screw 12 is integrally mounted on the pulley 38. The ball screw 12 is threaded with a ball nut 13.
The ball nut 13 is fixed to a plate 43, which is movable along a guide bar 44 in a direction indicated by an arrow A. A screw position detector 23 detects a screw position. The rotational speed of the injection motor 11 is detected by a speed detector 45.
The plate 13 is formed integrally with a plate through a load cell 18. A screw 20 is rotatably mounted on the plate 46. The plates 43 and 46 are movable along the guide bar 44 in the direction indicated by the arrow A.
A screw rotating motor 48 is mounted on the plate The rotation generated by the screw rotating motor 48 is transmitted to the screw 20 through a pulley 49, a timing belt 50 and then a pulley 51. The rotational speed of the screw rotating motor 48 is detected by a speed detector 53.
Reference numeral 52 denotes a hopper for accommodating resin pellets, reference numeral 54 denotes a heating cylinder, and reference numeral 58 denotes an injection nozzle through which the resin is injected.
The operation of the above-described electric injection molding machine will be described below.
In the metering process, the rotation generated by driving the screw rotating motor 48 is transmitted to the screw 20 through the pulley 49, the timing belt 50 and then the pulley 51. Consequently, the screw 20 retracts and the resin supplied from the hopper 52 is melted and accumulated in the front end portion of the heating cylinder 54.
At that time, a rotation is applied to the ball screw 12 through the pulley 36, the timing belt 37 and then the pulley 38 to apply a back pressure to the screw through the ball nut 13, the plate 43, the load cell 18 and then the plate 46. After this back pressure has reached an adequate value, the amount of resin accumulated in the front end portion of the heating cylinder 54 increases gradually due to retraction of the screw 20.
At that time, the screw position detector 23 detects the screw position by detecting the position of the plate 43. When the screw 20 has retracted to a preset position, the operation of the screw rotating motor 48 and the operation of the injection motor 11 are halted, thereby completing the metering process.
In a subsequent injection process, the injection motor 11 is controlled according to the speed setting value under the condition that the screw rotating motor 48 is not rotated. Thus, the screw 20 is advanced without being rotated to inject the resin into the cavity of the mold (not shown) from the injection nozzle 58.
When the reaction force detected by the load cell 18 provided between the plates 43 and 46 exceeds the set value, filling is stopped and the dwell process starts. In the dwelling process, the load cell 18 detects the dwell pressure. Therefore, the injection motor 11 is controlled such that the dwell pressure equals the set value.
FIG. 4 is a block diagram of a control device for another type of conventional electric injection molding machine.
In FIG. 4, reference numeral 11 denotes an injection motor, and reference numeral 18 denotes a load cell for detecting the dwell pressure after filling is completed. The dwell pressure detected by the load cell 18 is fed back to a subtracter 61 as a pressure detection signal e. The subtracter 61 subtracts the pressure detection signal e from a pressure setting signal f to obtain a pressure deviation signal g. The pressure deviation signal g is supplied to a compensator 63.
The resultant signal of the compensator 63 is output to a servo amplifier 64 as a speed instruction signal h. The servo amplifier 64 controls armature current j of the injection motor 11 such that the speed instruction signal h equals rotational speed detection signal i detected by the speed detector 45.
Thus, when the screw 20 (FIG. 3) presses against the resin in the heating cylinder 54 and cavity (not shown), a reaction force is received by the screw 20 as a dwell pressure. The load cell 18 detects this dwell pressure as the pressure detection signal e. Control is performed such that the pressure detection signal e equals the pressure setting signal f.
However, in the control device for another type of conventional electric injection molding machine, overshoot may occur or a steady-state deviation may increase at a changing point of the pressure setting signal f.
FIG. 5 shows the waveform of the pressure setting signal and that of the speed detection signal in the dwell process.
In FIG. 5, t0 through t3 represent points along a time axis t, and P1 through P3 represent the values of the pressure setting signal f set at time points t0, t1 and t2, respectively.
The pressure detection signal e in the injection process has a waveform determined by both the screw speed and the load pressure. When the above-described pressure control operation is initiated in the dwell process, overshoot may occur or a steady-state deviation may increase at the changing point of the pressure setting signal f.
Hence, in order to prevent occurrence of overshoot, gain may be reduced. However, this increases the steady-state deviation or slows down response.
Conversely, an increase in the gain, which is conducted in order to reduce the steady-state deviation and thereby speed up response, makes the control system unstable in a transient state due to influence of the non-linearity of the control system, generating vibrations or overshoot.